1. Field of the Invention
The present invention relates to tricyclic heteroaromatic compounds and their methods of use and, more particularly, to tricyclic heteroaromatic compounds that inhibit receptor tyrosine kinase(s), dihydrofolate reductase, thymidylate synthase and/or dihydroorotate dehydrogenase activity so as to exert cytostatic and cytotoxic action on tumor cells and other proliferative diseases and disorders.
2. Description of the Prior Art
The formation of new blood vessels from existing vasculature is termed angiogenesis. Angiogenesis plays a crucial role in the growth and metastasis of solid tumors. Solid tumors require angiogenesis to grow beyond 1-2 mm in diameter and metastasis requires the presence of blood vessels to allow access to the circulation and to form tumors at distal sites to the primary tumor. Angiogenesis and metastasis contribute to the poor prognosis in patients with angiogenic solid tumors. Thus, agents that inhibit the angiogenic process have afforded new paradigms for the treatment of tumors.
Angiogenesis primarily is a receptor-mediated process by growth factors that cause signal transduction, for the most part, by receptor tyrosine kinases (RTKs). RTKs consist of families of growth factor receptors such as vascular endothelial growth factor receptor (VEGFR), epidermal growth factor receptor (EGFR), platelet-derived growth factor receptor (PDGFR) and fibroblast growth factor receptor (FGFR). Aberrant expression or overexpression of EGFR and PDGFR, both of which are directly or indirectly involved in angiogenesis, have been implicated in the development, progression and aggressiveness of a variety of solid tumors. These include head and neck cancers (Shin, D. M. et al., Cancer Res., 54:3153-3159, 1994), non-small cell lung cancer (Tateishi, M. et al., Cancer Res., 50:7077-7080, 1990; Gorgoulis, V. et al., Anticancer Res., 12:1183-1187, 1992), glial tumors (Fleming, T. P. et al., Cancer Res., 52:4550-4553, 1992) and glioblastomas (Fleming, T. P. et al., Cancer Res., 52:4550-4553, 1992; Maxwell, M. et al., J. Clin. Invest., 86:131-140, 1990; Hermanson, M. et al., Cancer Res., 52:3213-3229, 1992).
Because RTKs are present in endothelial cells (VEGFR, PDGFR), tumor cells (FGFR, PDGFR) and pericytes/smooth muscle cells (FGFR, PDGFR), inhibition of more than one RTK may provide synergistic inhibitory effects against solid tumors. Thus, RTKs are attractive targets for cancer chemotherapeutic agents.
The importance of multiple RTK inhibition in angiogenesis is well recognized for the treatment of diseases such as cancer and macular degeneration, with multiple compounds in clinical use and several currently in various phases of clinical trials. These compounds, however, only are cytostatic, stopping the growth of tumors by blocking the angiogenesis pathway, and thus depriving tumors of the nutrition they need to grow. Hence, the antiangiogenic effect of these compounds does not kill tumor cells. To kill tumor cells, an additional cytotoxic effect is necessary. This cytotoxic effect can be provided by existing cancer chemotherapeutic agents. Thus, a variety of RTK inhibitors that are antiangiogenic and cytostatic have been combined with existing cancer chemotherapeutic agents that are cytotoxic, such as dihydrofolate reductase (DHFR) and thymidylate synthase (TS) inhibitors, in which DHFR and/or TS is the cytotoxic target. Preclinical and clinical trials of such combinations and other similar combinations have provided synergistic effects that are superior to either drug alone.
Preclinical studies also have shown that inhibition of multiple RTKs has shown an increase in survival of mice (Shaheen, R. M. et al., Cancer Res., 61:1464-1468, 2001). Thus, it is believed that the use of RTK inhibitors along with cytotoxic or conventional cancer chemotherapeutic agents and/or radiation enhances the efficacy of overall antitumor therapy and prevents regrowth following cessation of therapy (Dancey, J. et al., Nat. Rev. Drug Discov., 2:296-313, 2003; Kerbel, R. et al., Nat. Rev. Cancer, 2:727-739, 2002).
RTKs generally are transmembrane receptors consisting of an extracellular growth factor binding domain, a hydrophobic transmembrane domain, and a cytoplasmic domain. The cytoplasmic domain contains regulatory regions and a catalytic tyrosine kinase domain with a binding site for both ATP and substrates allowing for autophosphorylation, which is critical for signal transduction and angiogenesis.
Recently, VEGFR-2 and PDGFR-β, two RTKs, have been implicated in controlling angiogenesis at two different stages of the angiogenic process. In addition, inhibition of VEGFR-2 and PDGFR-β with two separate inhibitors, SU5416 and SU6668, respectively, has been shown to produce a synergistic effect in early stage as well as late stage pancreatic islet cancer in mouse models by attacking the angiogenic process at two different sites (Bergers, G. et al., J. Clin. Invest., 111:1287-1295, 2003; Erber, R. et al., FASEB J., 18:338-340, 2004).
DHFR carries out the reduction of dihydrofolate to tetrahydrofolate (THF), which is utilized by serinehydroxymethyltransferase to produce 5,10-methylene-tetrahydrofolate (5,10-CH2THF). The cofactor 5,10-CH2THF serves as the source of the methyl group in the conversion of deoxyuridine monophosphate to thymidylate catalyzed by thymidylate synthase (TS). Both TS and DHFR inhibitors are well-established cytotoxic agents used in cancer chemotherapy (Gangjee, A. et al., Curr. Pharm. Des., 2:263-280, 1996). Methotrexate and trimetrexate are examples of such classical and non-classical antifolates, respectively, and 5-fluorouracil and pemetrexed are examples of TS inhibitors used clinically.
U.S. Pat. No. 5,679,683 discloses 4-substituted amino benzothieno[3,2-d]pyrimidine and 4-substituted pyrrolo[2,3-d]pyrimidine inhibitors of epidermal growth factor receptor family of tyrosines.
Showalter, H. D. H. et al. (J. Med. Chem., 42:5464-5474, 1999) disclose 6,5,6-tricyclic benzothienol[3,2-d]pyrimidines and pyrimido[5,4-b]- and -[4,5-b]indoles as inhibitors of epidermal growth factor receptor tyrosine kinases.
PCT published patent application No. WO2005/042500 discloses arylindenopyridines and arylindenopyridines and their use as an adenosine a2a receptor antagonist.
Gangjee, A. et al. (Bioorganic and Medicinal Chem., 13:5475-5491. 2005) disclose 5-substituted, 2,4-diaminofuro[2,3-d]pyrimidines as multireceptor tyrosine kinase and dihydrofolate reductase inhibitors with antiangiogenic and antitumor activity.
In general, it is highly desirable to develop new anti-angiogenic and anti-tumor compounds which inhibit both the formation of new blood vessels as well as selectively kill tumor cells. There is a need, therefore, for single compounds which provide the desired enzyme inhibition to achieve both cytostatic and cytotoxic activity with a high degree of selectivity and low toxicity.